FIELD OF THE INVENTION
The invention relates to an integratable current source circuit for generating an output current proportional to an input current.
Such current source circuits typically include current mirrors of the kind which are known, for instance, from a book entitled: Analysis and Design of Analog Integrated Circuits, by Paul R. Gray, published by John Wiley and Sons, 1984, pp. 234-239. In the simplest form of the current mirrors, a diode, or an input transistor wired to make a diode, is triggered in the conducting direction with an input current, and the voltage dropping across the diode triggers an output transistor, by which an output current proportional to the input current is impressed. Improvements to that simple principle of circuitry provide the connection of a resistor in the emitter line of the output transistor or the replacement of the diode by a configuration of two transistors, with the input current being carried through the collector-to-emitter path of one of the two transistors, and the collector and base of the one transistor being connected to the base and emitter of the other transistor, which on its collector side is connected to a supply potential. The voltage for triggering the output transistor is tapped between the emitter and the base of the first transistor.
With such current mirrors, satisfactory results are obtained in integrated circuits when npn transistors are used, since the npn transistors being used typically exhibit high current amplification with little deviation from one another, so that deviations between the output and input currents are kept slight. Conversely, in current sources, pnp transistors are used, and their current amplification is substantially less while their deviation in terms of current amplification is substantially greater from one to another than in npn transistors. Therefore there is a considerably greater difference between the input and output currents, making the overall precision substantially less.